1. Field of the Invention
The present invention relates to ladders and in particular to combination step and extension ladders which may be folded and unfolded from a step ladder configuration to a straight extension ladder configuration and vice versa.
2. The Prior Art
Ladders are commonly used for a variety of applications and are of two general types. One type is a folding ladder, commonly called a step ladder, which is self-supporting. Step ladders are typically used for such tasks as pruning, painting ceilings or other similar tasks where it may be imossible to lean the ladder against a structure for support. Step ladders typically have wide, flat rungs which enhance the stability and comfort when climbing or standing on the ladder.
The other type of ladder which is well known is the straight extension ladder. This type of ladder is simply leaned against a wall or some other structure when standing or climbing on the ladder. Such ladders often include an extensible segment which can be used to telescopically extend the length of the ladder as desired.
Ladders which are constructed so that they may be used as both step ladders and as straight extension ladders have long been known in the art. For example, see U.S. Pat. Nos. 594,303, 1,100,823 and 3,912,043. Typically, such ladders are constructed with hinges in the middle of the side rails. The hinges permit the ladder to be folded into a step ladder configuration or unfolded into a straight extension ladder configuration. As will be readily appreciated, such ladders (hereinafter designated "combination step and extension ladders") are very versatile and they combine the desirable features of both types of ladders.
However, combination step and extension ladders have presented problems with respect to the type of ladder rungs which may be used with such ladders. For example, one type of ladder rung which is often used with combination step and extension ladders is a dowel-shaped rung. See, e.g., U.S. Pat. No. 1,100,823. Dowel-shaped ladder rungs have the advantage of permitting the user to climb up either side of the ladder when it is used in the step ladder configuration. However, dowel-shaped rungs are much more difficult to climb or stand on than wide, flat rungs.
Combination step and extension ladders have also used wide, flat ladder rungs. See, e.g., U.S. Pat. No. 594,303. Typically, flat ladder rungs are angularly oriented with respect to the side rails so that when the ladder is leaned up against a wall or otherwise stood in the upright position, the flat rungs will present an essentially horizontal stepping surface. However, when folding a combination step and extension ladder having flat ladder rungs into the step ladder configuration, a user can only climb up one side of the step ladder since the ladder rungs on the other side will not present a horizontal stepping surface but will instead form an acute angle.
In order to avoid the problems mentioned with the types of ladder rungs described above, more recent combination step and extension ladders have used a combination of both flat ladder rungs and dowel-shaped rungs. See, e.g., U.S. Pat. No. 3,912,043. Thus, when folded into the step ladder configuration, a user may climb up either side of the step ladder, using either the side with dowel-shaped rungs or the side with flat rungs. Although this increases the versatility of a combination step and extension ladder, it will nevertheless be appreciated that in many applications the problems attendant with dowel-shaped rungs are not overcome with this type of ladder. Furthermore, when unfolded so as to be used as a straight extension ladder, the ladder must be stood on a particular end of the side rails to ensure that the flat ladder rungs will present a horizontal stepping surface.
It would therefore be an improvement in the art to provide a combination step and extension ladder having ladder rungs with flat stepping surfaces which are always essentially horizontally oriented regardless of which configuration the ladder is in and regardless of which end the ladder stands on. It would also be advantageous to provide a combination step and extension ladder which may be extended to increase the height of the ladder in both the straight extension ladder configuration and in the step ladder configuration.